A number of lasers, such as slab and rod lasers, are designed to produce output pulses having a high average output power, such as 1,000-10,000 W for a 100-1000 KHz diode pumped Nd:YAG slab laser. High levels of output power are required in a number of applications including laser radar, mine detection, welding, material processing, surface coating, isotope separation and x-ray lithography, among others. In order to obtain such high power levels, a primary laser, such as a slab or a rod laser, can be pumped by a laser pump source, such as an array of semiconductor laser diodes. The laser pump source must also operate at relatively high power levels and either at relatively high pulse repetition rates or continuously in order to generate the necessary power to excite the primary laser.
In generating pulses having a relatively high average output power and a relatively high repetition rate, the laser pump source generates a significant amount of heat which elevates the temperature of the laser pump source in the absence of external cooling. The amount of heat generated by conventional lasers is relatively large. For example, the heat generated by a laser can be approximated by the difference between the power input to the laser and the output power received from the laser. Typically, the heat generated by a conventional laser is approximately 50%-60% of the input power.
Lasers, such as semiconductor laser diode arrays, however, typically have a maximum operating temperature above which the operation of the laser can be unreliable. In addition, operation of a laser, such as a semiconductor laser diode array, at an elevated temperature generally reduces the effective lifetime of the laser even though such temperatures may be below the maximum operating temperature. For example, operation of a semiconductor laser diode array at an elevated temperature can damage the emitting facet of the laser diode array, thereby impairing its performance.
Consequently, a number of techniques have been developed to cool lasers and, in particular, to cool semiconductor lasers, such as semiconductor lasers which pump a primary laser. For example, semiconductor pump lasers generally include a plurality of linear arrays of laser diodes which are arranged in a two-dimensional laser diode array. One conventional technique for cooling semiconductor pump lasers is back plane cooling. As known by those skilled in the art, back plane cooling is typically provided by a heatsink which is in thermal contact with the semiconductor pump laser and which draws heat from the semiconductor pump laser. The heatsink is also preferably electrically conductive to provide electrical continuity between the plurality of linear laser diode arrays of the semiconductor pump laser. For example, the heatsink can be fabricated from copper which is electrically conductive and which has a relatively low thermal impedance.
In order to improve its heat dissipation, the heatsink generally includes a plurality of microchannels defined therein which are adapted to carry fluid, such as water. The fluid absorbs heat from the heatsink and, in turn, from the semiconductor pump laser, thereby cooling the semiconductor pump laser. Typically, the microchannels defined in the heatsink are in fluid communication with an external cooler or chiller, such as a radiator or other type of heat exchanger. The external cooler removes heat from the circulating fluid to thereby reduce the temperature of the fluid. Accordingly, the fluid can be recirculated to further cool the semiconductor pump laser.
Back plane cooling suffers from several deficiencies, however, including the necessity for a secure and thermally conductive attachment between the heatsink and the semiconductor pump laser. For example, thermal inefficiencies are incurred in the conduction of heat from the plurality of laser diode junctions of a semiconductor laser array at which the heat is generated to the heatsink. Further, the fluid flow must be strictly confined within the microchannels since the semiconductor pump laser diode is typically electrically activated.
Thus, in order to reduce the chances of an electrical fault or a short circuit, heatsinks comprised of materials which are both electrically insulating and thermally conductive, such as Beryllium Oxide (BeO), have been employed. Since BeO is electrically insulating, however, the surfaces of the BeO heatsinks must generally be patterned in metal in order to provide sufficient electrical continuity between the plurality of stacked linear laser diode arrays which form the two-dimensional laser diode array. The metal pattern formed on the BeO heatsinks produces a parasitic series resistance which, although relatively small, is significantly larger than the resistance of a comparable copper heatsink. In addition, a BeO heatsink conducts heat much less efficiently than a copper heatsink having comparable dimensions. Thus, heatsinks formed of insulating materials, such as BeO, generally suffer from reductions in both their thermal performance and their electrical efficiency in comparison with comparable copper heatsinks.
Another technique for providing back plane cooling of a semiconductor pump laser includes a silicon carrier having a front surface to which the semiconductor pump laser is bonded. The front surface of the silicon carrier can include turning mirrors, integrally fabricated thereon, to reflect laser light emitted by the semiconductor pump laser upward and away from the surface of the silicon carrier. The silicon carrier can also include an integral microchannel structure defined on a rear surface, opposite the front surface. As described above, the microchannel structure is adapted to carry fluid, such as water, which cools the silicon carrier and, in turn, cools the semiconductor pump array.
In order to provide sufficient electrical continuity between the semiconductor pump laser and the silicon carrier, however, the upper surface of the silicon carrier must generally either be metalized or doped to degeneracy. As described above, this metallization or degenerative doping, while providing electrical continuity between the plurality of linear laser diode arrays, increases the parasitic series resistance of the silicon carrier as compared to a copper heatsink of comparable dimensions. In addition, a silicon carrier generally has a lower thermal conductivity than a comparable copper heatsink.